The present invention relates to a high throughput screening device for combinatorial chemistry, particularly, an array fuel cell (FC) that utilizes a one or more common electrode flow fields and an array electrode flow field that permits the evaluation of 25 fuel cell electro-catalyst surfaces simultaneously. The catalysts can be anode or cathode electro-catalyst candidates. Variations of catalysts compositions and/or methods of preparation can be evaluated in a high throughput mode.
Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy. A fuel cell, although having components and characteristics similar to those of a typical battery, differs in several respects. The battery is an energy storage device. The maximum energy available is determined by the amount of chemical reactant stored within the battery itself. The battery will cease to produce electrical energy when the chemical reactants are consumed (i.e., discharged). In a secondary battery, recharging regenerates the reactants, which involves putting energy into the battery from an external source. The fuel cell, on the other hand, is an energy conversion device that theoretically has the capability of producing electrical energy for as long as the fuel and oxidant are supplied to the electrodes. In reality, degradation of catalyst performance, corrosion, and/or malfunction of components limit the practical operating life of fuel cells.
Fuel cells have been used on the space shuttle for a couple of decades. However, the fuel used in the fuel cells used on the space shuttle is pure, liquid hydrogen. Liquid hydrogen is expensive and requires cryogenics not practical for consumer use.
Gasoline, diesel and methane and alcohols are fuels that are practical for consumer use. However, gasoline, diesel, methane do not have adequate electrochemical reactivity to be used directly in state-of-the-art PEFCs for high power applications. A catalytic-chemical fuel processor (reformer) is required to convert these fuels to hydrogen-rich fuel gases. The reforming process yields H2 diluted with CO2, and low levels of CO. Within the operating temperature (T) range of polymer electrolyte fuel cells, the reformate prior to the water gas shift (WGS) and the preferential oxidation (PROX) reactor contains CO at the pph level, enough to shut down a Pt alloy catalyst. The WGS output contains about 1% CO, still enough to shut down the anode. A PROX unit is used to further reduce the CO content to the approximately 10-ppm tolerance limit of a typical anode catalyst (PtRu). The water-gas-shift reactor and the preferential oxidation unit are large units that reduce the overall power density of the fuel cell system. The development of CO tolerant anode catalysts would obviate the need for the PROX and WGS units and thus permit the design of more compact and efficient system. The development of superior anodes requires the discovery of new catalytic materials.
Another type of fuel cell is the direct methanol fuel cell (DMFC). The DMFC differs from reformate fuel cells because the fuel (methanol) is delivered directly to the anode catalytic surface, without prior reforming. A low temperatures (below 100xc2x0 C.) methanol is the only liquid fuel that is sufficiently reactive for anode surfaces. An intermediate chemical that is formed during the oxidation of methanol is carbon monoxide (CO). CO poisons the DMFC anode catalytic surface in much the same way that CO poisons the surface of reformate anodes. Thus DMFCs require CO tolerant anodes as do reformate fuel cells. The development of superior DMFC anodes requires the development of better catalysts.
Progress in the area of catalyst discovery has been slow for a number of reasons. There are two electrodes in the fuel cell, the anode where the fuel is oxidized and the cathode where oxygen from air is reduced. A fuel cell, which only has one membrane electrode assembly (MEA), is termed a single cell. In catalysis work, only single cell assemblies are used for comparing catalysts.
If anode catalysts are being compared, it is important to ensure that the cathode electrode is not a variable (i.e. that the performance of the cathode from one test to another does not vary). It is important that the common electrode (the cathode in the case of searching for anode catalysts) is invariant so that changes in performance can be ascribed entirely to the anode performance. If the cathode performance varies from test to test, it becomes impossible to compare anode catalysts. Fuel cell systems are very complicated. In practice it is difficult to insure that the cathode is invariant.
Another issue is conditioning. After a catalyst layer has been inserted into the fuel cell, the layer must be conditioned prior to attempts to make steady state measurement. Conditioning is a process that usually involves operating the fuel cell for a period of time at a selected cell voltage or current. The effects of conditioning are not well understood. During the conditioning process, the catalysts may be undergoing morphological and/or chemical changes. Conditioning may also be related to wetting of the metal catalyst layer with the polymer electrolyte. The conditioning process can take from 1 to 3 days. For initial screening of a catalyst, at least three days of data acquisition are required after the conditioning process. Thus with a single cell, the testing of one catalyst requires 4 to six days of conditioning and data acquisition. In order to include a statement of uncertainty with the final result, the catalysts should be repetitively tested with 4 to 6 sample of the catalyst. Thus the testing one catalyst with statistical reliability requires at least 25 days of continuous test stand operation. The comparison of 5 catalysts reliably would take 125 days of testing. Preparation of the electrodes would require about 4 additional days. 129 days of test stand operation including weekends off would require almost 200 days. These timelines are difficult ranges for small businesses testing catalysts with short delivery times. Finally, the above type to testing on single cell systems is not reliable because each single cell test requires the assumption that the common electrode is performing the same for each and every test. The likelihood of 125 days of test stand operation having uniform cathode performance is very low. Thus it is likely that the time required for reliable testing should be about doubled. Thus, it can take over a year to test and compare 5 catalysts and this timeframe would not permit lifetime studies.
It has been over 30 years since PtRu was found to be the best but inadequate catalysts for DMFCs. If new catalysts are to be discovered, new high throughput screening methods will be required and that is the motivation for this invention.
An object of the present invention is a single cell fuel cell assembly having standard graphite flow field block for one electrode and an array flow field block for contact to the side of the membrane electrode assembly incorporating the array of catalyst to be simultaneously tested.
An embodiment of this invention is a high throughput screening device for combinatorial chemistry, comprising a membrane electrode assembly, one or more common electrodes and an array of sensor electrodes wherein a total cross-sectional area of the one or more common electrodes is greater than a sum of the cross-sectional areas of the sensor electrodes and the device does not require a movement of any electrode during data acquisition. Another embodiment is a high throughput screening device for combinatorial chemistry, comprising a membrane electrode assembly, one or more common electrodes and an array of sensor electrodes wherein a total cross-sectional area of the one or more common electrodes is greater than a sum of the cross-sectional areas of the sensor electrodes and the array of sensor electrodes are operated simultaneously.
The array of sensor electrodes are capable of being operated simultaneously in a fuel cell. The device could further comprise a catalyst. The catalyst could be a fuel cell catalyst. The catalyst could be applied to a carbon diffusion layer or to a membrane. The membrane electrode assembly could comprise an electrolyte layer and two catalyst layers. The electrolyte layer could be a polymer membrane. The device could further comprise a gas diffusion layer and a flow field block. The device could further comprise a current follower and a potential follower. The sensor electrode could comprise graphite. The membrane electrode assembly could comprise an electronically insulating proton conductor.
Additional advantages of this invention would become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiments of this invention are shown and described, simply by way of illustration of the best mode contemplated for carrying out this invention. As would be realized, this invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.